Safety device for use with prestressing cables

ABSTRACT

Safety equipment for use with prestressing cables as used in civil engineering and building structures, the safety equipment having two principle parts adapted to fit over a cable at a point where the cable interfaces with anchoring members or walls, the safety equipment configured to take over from original anchoring elements in the event of a rupture of the cable.

The present invention relates to a method for protecting civilengineering works or others civil engineering structures and buildingstructures including stretched external prestressing cables and/orsupporting cables and/or tension rods. It also relates to the safetyequipment used for implementing the process.

BACKGROUND OF THE INVENTION

The civil engineering works such as bridges, include reinforcementsintegrated directly to the concrete. They also include externalreinforcements, not integrated to the concrete properly speaking, formedof big tension cables also called prestressing cables. These cables arestretched between structure elements of the civil engineering works.

These external prestressing cables may have lengths of several tens ofmeters. They are stretched and anchored on elements of the civilengineering works which serve as stops and they may also run throughother elements of the civil engineering works such as walls orpartitions.

The ends of these cables include anchoring means which enable to keepthe initial prestressing tension.

These cables are generally well protected but it is not seldom to noticeruptures whereof the causes are multiple: corrosion, overload, fatigue.

The sudden rupture of a cable may cause major disorders on its closeenvironment, i.e. the structure of the civil engineering works and alsothe neighboring cables for instance.

Between the anchoring members, the cables running through the elementsof the civil engineering works; orifices are provided in these elementsor walls and the cables are guided by the reservation tubes dedicated tothat end during the construction.

The present invention offers a method which enables to prevent theconsequences of accidental ruptures of these cables.

SUMMARY OF THE INVENTION

The protection method according to the invention consists in installingat the mouthpiece of each passage orifice of the cable in the structureelement, a safety equipment which is arranged for degrading irreversiblyand quasi instantly, the energy released by the relaxed cable whenruptured.

This safety equipment somehow takes over from the primary anchorings ofthe cable and it may be provided for absorbing total relaxation of thecable, thereby mastering each section of the broken cable.

This method consists in fact in positioning on the cable(s), a safetyequipment especially designed for ancient existing civil engineeringworks, and in particular for dressing the cable(s) by dint of anequipment which comprises parts liable to be assembled in situ at alater stage, for enveloping the cable or extend of both sidessymmetrically.

According to a variation, the method consists in positioning theequipment or at least one portion of said equipment, simultaneously tothe realization of the civil engineering works and to the installationof the corresponding cable, by mounting said equipment on the cable ofboth sides of each orifice.

The invention relates also the safety equipment used for implementingthe protection method, which equipment is composed: —of a part attachedto the cable, distally from the mouthpiece of the passage orifice ofsaid cable in the structure element and, between this part and saidmouthpiece, of at least one energy absorption device, whereof the lengthis suited to the lengthening of the cable where tension is appliedthereto, and whereof the absorption capacity is programmed to enable theoperation thereof with a controlled load having a maximum value smallerthan the adherence load and friction rupture of the attachment of saidpart on the cable.

Still according to the invention, the part attached to the cable iscomposed of two semi-shells or jaws made of steel, aluminium orcomposite material; these semi-shells are assembled and clamped on thecable by any appropriate means, screws, bolts and the clamping pressureis adapted to avoid damaging components of the cable, i.e. its sheath,strands and slurries.

According to another arrangement of the invention, the internal surfaceof the shells or jaws, in contact with the periphery of the cable, isrough for better anchoring thereof to said cable; this internal surfacemay for instance include scores obtained by machining a thread whereofthe pitch is for instance 1 to 3 mm.

Still according to the invention, the energy absorption devicecorresponds to the device object of the patent application FR-02 05105.This device detailed thereunder is in the form of an elongated metalgutter whereof the section is in the form of a U or an omega, in orderto wrap partially the cable, and the flanks of this gutter are connectedtogether by tension rods and are maintained apart by spacers.

Still according to the invention, the energy absorption device consists,as in the patent application aforementioned, of a folded metal sheetincluding a base and two lateral sides slightly convergent whereof theupper rims, which are parallel to one another, are fitted with orificesfor letting tension rods through.

According to another arrangement of the invention, the lateral sides ofthis absorbing device are in the form of a trapezoid and its end planedescribes an angle with the plane of its portion forming the base, inorder to initiate gradually the deformation of the absorbing device,starting from the ridge of said base.

According to a variation, the absorbing device is in the form of astructure whereof the section is closed or in the form of any otherstructure enabling calibrated energy absorption.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be detailed further using the following descriptionand the appended drawings, given for exemplification purposes, wherein:

FIG. 1 represents, schematically, a portion of civil engineering worksmade of armoured concrete and an external stretched prestressing cable,protected by the method according to the invention;

FIG. 2 represents the safety equipment mounted on a cable;

FIG. 3 represents, seen from the end thereof, the energy absorbingdevice and the means which enable the assembly thereof on the cable;

FIG. 4 represents, laterally, the energy absorbing device;

FIG. 5 represents, at enlarged scale, both semi-shells or jaws intendedto be fixed to the cable;

FIG. 6 represents, seen from below, the energy absorbing device deformedfurther to a ruptured cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows very schematically, a portion of civil engineering workssuch as for instance a portion of bridge, which includes an upper floor1, a lower floor 2 and structure elements composed for instance of wallsmarked 3, 4, 5 and 6.

A cable 7 serving as an external prestressing cable is stretched betweenthe walls 3 and 6, maintained in stretched condition by anchoring means8 and 9 disposed at the walls 3 and 6 respectively, which serve asstops.

On this cable 7, there can be noted on each side of the walls anequipment 10 which corresponds to the safety equipment according to theinvention.

This safety equipment 10, detailed on the following figures, comprises apart 11 forming the hammer, attached to the cable 7 and a device 12enabling to absorb the energy.

The cable 7 runs through the different walls 3, 4, 5 and 6. It is guidedinto orifices 13, 14, 15 and 16 and in particular into reservation tubesprovided during the construction of the works in the walls 3, 4, 5 and6. There can be noted, FIG. 1, that safety equipments 10 are displacedat each mouthpiece of the orifices.

Thus, in case of ruptured cable 7, regardless of the place, between thewalls 3 and 4, or 4 and 5, or 5 and 6, the safety equipments 10 play apart for absorbing the energy resulting from the relaxed cable.

FIG. 2 shows a safety equipment composed of a part 11 attached to thecable 7 and an absorbing device 12 which clamps said cable. Thisabsorbing device corresponds to that which was an object of the patentapplication FR-02 05105.

This device 12 is detailed thereunder. It is in the form of a U- oromega-shaped gutter. This gutter is shaped from a folded metal sheet. Itcomprises a base 17 and lateral walls 18. The base 17 is flat,rectangular in shape; the lateral walls 18 are also flat and include attheir upper section, a rim 19. This rim 19 is provided to enable theinstallation of tension rods 20 which connect said walls, closing thegutter on the cable 7. These tension rods 20 are formed of bolts forinstance and the space between the rims 19 of the walls 18, isestablished by means of spacers 21 in the form of tubes, mounted on saidtension rods 20.

FIG. 3 shows the absorbing device 12 during installation on the cable 7.This absorbing device has a section which is in the form of a U or anomega, enveloping the cable 7. The walls 18 are slightly tilted withrespect to the base 17, tightening upwards. The upper rims 19 of thewalls 18 enable the installation of the tension rods 20 and theirspacing is calibrated by means of tubular spacers 21.

The absorbing device is mounted to slide on the cable 7.

FIG. 4 shows the absorbing device 12 seen laterally. Its lateral walls18 are in the form of a rectangular trapezoid. On one side, the end 22of the absorber is perpendicular to the base 17; this end 22 is incontact with the part 11 forming the hammer. The other end 23 of theabsorber is tilted, forming with the base 17 an angle smaller than 90°of the order of 80° for instance.

This end 23 corresponds to the end which is in contact with themouthpiece letting through the cable at the walls 3, 4, 5 or 6. In fact,it is the ridge 24 of the base 17 which is in contact with the walls, inorder to initiate gradually the deformation of the absorber duringoperation.

The tilt of this end 23 enables gradual increase in the deformation loadof the absorber 12, when the cable breaks off, and to crush the absorberby the part 11 which serves as a hammer, against the corresponding wall.

There can be noted, still on FIG. 4, disposed in the upper rim 19 of thelateral walls 18, the orifices 25 which let through the tension rods 20.These orifices 25 are for instance spaced regularly over the length ofthe absorbing device.

This absorber 12 is made out of a stainless steel sheet or any othermetal, composite or plastic material. For informative purposes, thissheet may have a thickness of the order of 3 to 4 mm, in the form of agutter to envelope a cable whereof the diameter is of the order of 90 mmfor instance.

The length of the absorber is chosen relative to the variation in lengthof the cable to be controlled when releasing it.

The deformation load of the absorber is chosen to be smaller than thefastening load of the part 11 on the cable 7.

This cable 7 is generally composed of several strands enveloped with asheath of thermoplastic material.

The part 11, as represented FIG. 5, is composed of two semi-shells 26assembled by screws or bolts 27. These semi-shells 26 have a cylindricalbore whereof the internal surface 28 is preferably rough for betteradherence to the periphery of the cable 7. This internal surface 28includes for instance scores corresponding to a kind of internal threadwhereof the pitch ranges between 1 and 3 mm for instance; this threadenables correct anchoring of each semi-shell on the cable, and to avoidany risks of slipping below a load value smaller than that whichcorresponds to the operation of the absorber 12.

Screws or bolts 27 run both semi-shells 26; their number and theclamping torque are determined in order to obtain an adherence load onthe cable which is vastly greater than the load necessary to deform theabsorbing device 12.

The part 11 may be made of steel, aluminium or of composite material,moulded or not.

FIG. 6 shows the absorbing device 12 seen from above, deformed under theaction of the part 11, between the latter and one of the walls 3, 4, 5or 6, after rupture of the cable 7. The absorber 12 deforms, bysuccessive folds, each fold being delineated by the tension rods 20.

According to the particular requirements, one may also provide theinstallation of several absorbers 12, displaced in series on the cable7, and separated by a free transition part 31 on the cable, in the formof a washer for instance.

Each absorber 12 may have an absorption capacity of the variation inlength of the cable corresponding to the length of this cable betweentwo adjacent walls. To absorb an elongation variation of the cablewhereof the length corresponds to double the space between two walls,one may provide the installation of two absorbing devices.

Quite often, the space between the cables is very small and the accessrelatively cumbersome as well, the form of a gutter of these absorbingdevices enables the installation thereof on of the cables of finishedcivil engineering works.

This form of gutter for the absorber 12 facilitates its assembly on thedifferent cables and its fixation by tension rods 20 is relativelysimple also. The rims 19 are clamped with a torque calibrated on thespacers 21, and not on the cable.

Similarly, the installation of the part 11 which serves as a hammer isrendered possible by the use of two semi-shells 26, one being run belowthe cable and the other positioned on the cable, both assembled by thescrews or bolts 27.

However, this safety equipment may also be mounted originally on thecables 7. In such a case, one may also use an absorbing device ofsection closed for instance made of metal material, composite materialor plastic material.

This safety equipment may also be used on any type of stretched cablesuch as for instance suspension posts, stay cables, supporting cables,anchoring tension rods. As the case may be, it may appear as detailedabove in the form of a gutter enveloping the cable or in the form ofabsorbing devices displaced on both sides of the cable, symmetrically.

This safety equipment is also suitable for any type of cable and inparticular multi-wire cables, multi-strand cables, sheathed cables andinjected or not with cement slurries or others.

1. A safety equipment for protecting a civil engineering work with anexternal pre-stressing cable guided into an orifice through an elementor wall of said civil engineering work, the safety equipment comprising:a hammer part (11) attached to the cable (7) and spaced from a mouth ofthe orifice; and at least one energy absorption device (12) interposedon the cable (7) between the hammer part (11) and the mouth of theorifice and having an absorption capacity configured to enable theabsorption device to undergo a deformation upon application of acontrollable load exerted by the cable upon an end of the device (12)via the hammer part (11), wherein a maximum value of the controllableload is smaller than a fastening load of said hammer part (11), saidfastening load being a load exerted by the cable upon the hammer part(11) causing the hammer part (11) to become unfastened from said cable,and wherein said deformation irreversibly and rapidly degrades an energyof the cable released with the application of the controllable load. 2.The safety equipment according to claim 1, wherein the hammer part (11)attached to the cable (7) comprises two semi-shells (26) assembled by atleast one fastener.
 3. The safety equipment according to claim 2,wherein the at least one fastener comprises any one of a groupconsisting of screws and bolts.
 4. A safety equipment for protectingcivil engineering work with an external pre-stressing cable guided intoan orifice through an element or wall of said civil engineering work,the safety equipment comprising: a hammer part (11) attached to thecable (7) and spaced from a mouth of the orifice; and at least oneenergy absorption device (12) interposed on the cable (7) between thehammer part (11) and the mouth of the orifice and having an absorptioncapacity configured to enable the absorption device to undergo adeformation upon application of a controllable load exerted by the cableupon the device (12) via the hammer part (11), wherein the energyabsorption device comprises an elongated metal gutter with lateral sides(18), wherein a maximum value of the controllable load is smaller than afastening load of said hammer part (11), said fastening load being aload exerted by the cable upon the hammer part (11) causing the hammerpart (11) to become unfastened from said cable, and wherein saiddeformation irreversibly and rapidly degrades an energy of the cablereleased with the application of the controllable load.
 5. The safetyequipment according to claim 4, wherein the elongated metal gutter ofthe energy absorbing device (12) is formed of a folded metal sheet, thefolded metal sheet having a base (17) and the lateral sides (18)extending from the base.
 6. The safety equipment according to claim 4,wherein the lateral sides (18) of the absorbing device (12) are eachformed as a rectangular trapezoid having a first end (22), a second end(23), and a base portion (17) including a ridge (24), the first end (22)being perpendicular to the base portion (17), and the second end (23)being tilted at an angle on the order of 80° with respect to said baseportion (17) in order to initiate gradually said deformation of theabsorbing device, the deformation starting from the ridge (24) of saidbase portion (17).
 7. The safety equipment according to claim 4, whereinthe hammer part (11) attached to the cable (7) comprises two semi-shells(26) assembled by at least one fastener.
 8. The safety equipmentaccording to claim 6, wherein the first end (22) is in contact with thehammer part (11).
 9. The safety equipment according to claim 5, whereinthe lateral sides (18) have upper rims (19), the upper rims fitted withorifices (25) and tension rods (20) extending through the orifices andacross a space between the upper rims to connect the lateral sides (18)together, the space between the upper rims being maintained by spacers(21) centered on said tension rods.